How to change the game control?
Indirect Control

I suggest to first exchange the keyboard sensors by message sensors. You do not even need to change your car controlling code. 



to



To make it working, you now need a "user input provider" (other object) to send the new messages.



Note: the True Level Triggering. This is necessary to continuous send the messages as long as the key is held. 

Now your game should act as it did before

File input

Now you can add a request provider that does not listen to the keyboard, but reads a file and sends according messages with the right timing. Be aware this is a continuous process. You should send a message (of on subject) just once per frame.

You need to process file operations via Python. How to do that is a different topic.